In some radio bands, for example, the 217-220 MHz VHF band, the U.S. Federal Communications Commission (FCC) grants non-exclusive primary licenses use of the band for various communication services, such as push-to-talk voice. Primary users (or PUs) pay for these non-exclusive rights with the expectation that they will not suffer interference by other users. The FCC also allows secondary users (or SUs) to use the same band and the same channel within the band on a ‘non-interfering’ or secondary basis. Namely, so long as the primary user is not using a channel within the band, a secondary user may use it. Similar policies are also practiced by frequency management authorities in foreign countries.
The FCC and similar agencies in foreign countries are anxious to find ways to enable expanded use of such radio frequency bands, without reducing the quality of service available to the licensed primary users. For secondary users, this band—and other similar bands—represent a cost-free band with excellent radio transmission properties for telemetry and other uses. Because secondary users must not interfere with primary users, complaints of interference from a primary user to the FCC may result in an administrative order from the FCC requiring that the secondary user move to another portion of the band or leave the band entirely.
This movement can be disruptive to the secondary user's service and can be expensive, especially if site visits, equipment modification, or exchange is required to implement the change. Therefore, a mechanism is needed by which a secondary-use radio may employ the band on a non-interfering basis and which will adapt the radio's frequency usage should new primary users appear. It should be noted that primary users (who are licensed) always have priority over secondary licensed users or secondary unlicensed users; there is no first-use channel frequency right for secondary users.
Interference within such a band can occur in at least two ways. First, the primary user and one or more secondary users may have occasion to use the same channel and significantly disrupt one another. Secondly, one or more secondary users may transmit briefly with enough power and for a sufficient duration to cause some unwanted effect on the primary user's radio, such as a burst of noise through it's speaker or loss of data. Both of these and other types of interference must be avoided.
Primary users typically are field personnel who use an assigned channel for occasional voice transmissions to coordinate their activities with a central site or with co-workers in a nearby locale. It is likely that band usage will increase in the future. The aforementioned disruption of service caused by the appearance of new primary users warrants the need for a communication technique that will effectively avoid interference by secondary users.
Because of the occasional use of the band by primary users, it has been difficult to directly observe what frequencies are being used. It has also been difficult to coordinate frequency use by monitoring applications for primary use because of administrative delays. A further difficulty is that manual coordination must be done continuously, since new primary users may appear at any time.
One currently employed approach for having secondary users share a frequency band is to use manual frequency coordination. In accordance with this scheme, an installer or maintainer of a secondary user of a narrowband or wideband radio selects a frequency that appears not to be used, and then manually adjusts the radio through physical controls, or through a management control channel (in-band or out-of-band). The frequency selection process is typically performed by examining primary user license applications, or through the use of an instrumented spectral survey. The installer or maintainer then selects a frequency with sufficient, contiguous unused bandwidth to support the application. One problem with this approach is that it is labor-intensive, and it is difficult to differentiate between primary users and other secondary users.
An additional problem with this frequency selection process is that a primary user may appear only occasionally or a new primary user may appear unexpectedly. Without a further means of frequency coordination, the secondary user may interfere with the primary user and become subject to an administrative order to change the frequency or exit the band, as noted above. Another problem with this approach is that, in a busy band, it may become difficult to find enough contiguous bandwidth to support a given application. Moreover, adapting manual frequency radios to new services that require different or varying bandwidths may be difficult or expensive, depending on how bandwidth of the radio is adjusted.
Another technique for frequency coordination is to monitor applications for primary use licenses. This solves the difficulty of differentiating primary users from secondary users, but suffers all the other problems associated with human monitoring of spectrum through instrumentation, described above.
A further approach for allowing secondary users to share a frequency band is through the use of agile frequency, narrowband or wideband radios with automatic frequency coordination. This type of radio automatically detects interference from other radios and adjusts its frequency. For example, if the radio detects a high bit error rate for a sustained period, it may begin scanning for other, available frequencies or use prescribed alternate frequencies. While this constitutes an improvement over manual frequency coordination processes, it suffers from some of the same issues associated with manual frequency coordination. For example, it may be difficult to find contiguous bandwidth for the application, it may be difficult to distinguish between primary and secondary users, or it may be difficult to adapt the radio to new services that require different or varying bandwidths. Another problem with this approach is that it interferes with primary users during the interference detection phase and possibly during the frequency change phase, depending upon implementation.
Still another approach for enabling secondary users to share a frequency band with primary users is through the use of a frequency hopping spread spectrum (FHSS) radio. This type of radio spreads the transmission over a very wide band via frequency hopping with low spectral density. The radio occupies the entire band, but operates at a level sufficiently close to the noise floor to minimize the effect that the transmission will have on narrowband or wideband radios. Such a radio intends to solve the frequency coordination problem of manual frequency or agile frequency radios described above, and has the added benefit of coding gain.
However, a problem with this type of radio is the fact that the spread spectrum modulation can produce unknown effects on narrowband and wideband radios, such as possible squelch circuit activation or other types noise caused by interference. Another problem is that above 1 GHz (such as in the 2.4 GHz band), it may be difficult to close the radio link at the distances needed for many applications, such as telemetry using cost-effective radio equipment. Another problem is that when two secondary (licensed/unlicensed) users are communicating, there may be a higher-powered, interfering primary user that is closer to the secondary user transmitter than the intended secondary user receiver. In this case, the power in the FHSS signal must be increased to overcome the strong interference, which increases interference to the primary user, and decreases the jamming margin. (This latter problem is often referred to as the ‘near/far’ problem.) An additional problem is the fact that FHSS inherently occupies all channels, including primary user channels; therefore, it is not clear how the Federal Communications Commission in the United States, and equivalent authorities in foreign countries, will view FHSS for secondary use.